1. Field of the Invention
The present invention relates to a system for finding the presence of a predictable signal in a noisy environment. More particularly, the present invention relates to a system for finding a temporally describable known signal in a noisy environment, such as in a radar system.
2. Discussion of the Related Art
One of the most common problems in signal processing is to determine when a signal of interest is present within a noisy environment. Of course, it is impossible to remove all noise. However, as the signal power becomes lower in relation to the noise, i.e., the signal-to-noise power ratio gets smaller, it is more and more difficult to determine if the signal of interest is present or not. A number of different processes have been utilized in order to help locate the presence of such a signal. However, all systems of this type necessarily have limits as to their ability to find a signal when the signal-to-noise ratio becomes small.
One particular area where this is important is in the context of radar. It is known what type of signal should be expected when aircraft are within the range of the radar system. However, it is important to locate the aircraft as soon as possible, even when the signal-to-noise ratio is very low. In modern technology, for example, stealth construction of aircraft effectively lowers the signal-to-noise ratio, so that it is more difficult to detect such aircraft. Also, it is important to detect incoming aircraft even when it is distant and the signal power is low, so that adequate warning time is given, especially if it is an enemy aircraft situation. In other words, detecting the aircraft while it is far away and still has a small signal could be a matter of life or death for defending installations. Whether the situation is in the context of radar or in other situations where a known signal is expected, such as sonar, communications, etc., a signal having a known pattern is expected and knowledge of this time varying pattern makes it possible to locate it even if a great deal of noise is present.
In the past, correlation techniques have been utilized in order to detect signals within a noisy environment. A matched filter implementation has commonly been used in order to help find such a signal. In the case of a time varying signal, the signal is delayed such as in a tapped delay line and samples are taken from the various taps so as to give a picture of the signal as it varies over time. Each of the taps or points on the signal is then multiplied with a vector of weights which in some fashion corresponds to the signal of interest. Thus in a simple fashion, if one expects the signal to have a certain shape, the weights will represent this same shape. These values can then be summed for all the different sampled points to give a correlation output which represents how close the sampled signal comes to a desired signal. If this output exceeds a given threshold it is determined that the signal is present.
This correlation system is very workable and appropriate for radar or other similar systems where a known pattern is being sought. However, such systems have limits, especially where the power of the signal being sought is lower than the noise and interference in which it is found. When it is desired to locate much lower power signals within a noisy environment this system fails.
Accordingly, the present invention provides a system for locating a signal of interest within a noisy environment.
The present invention further provides a system for locating a signal of interest within a noisy environment using a correlation technique.
The present invention further provides a method for detecting a signal of interest in a noisy environment using a parallel reference method.
The present invention further provides a method and apparatus for detecting a signal of interest in a radar situation using a parallel reference technique.
The present invention further provides a method and apparatus for detecting a signal of interest in a noisy background using a parallel reference technique and a Chi-Square Goodness of Fit test.
The present invention further provides a method and apparatus for detecting a signal of interest in a noisy background using a parallel reference technique and a Mean Square Error test.
This is accomplished in the present invention by providing a parallel reference system for making it easier to see the desired signal. A pattern representing a signal of interest is cross correlated with the incoming signal and is also autocorrelated. The two signals are then subjected to various mathematical operations in a feature generator so as to produce reference and observed signals which are then compared in a Chi Square Goodness of Fit test, a Mean Square Error test, or some other test for closeness of fit. If the observed signal is similar to the reference signal according to this test, a low value is given. This value is compared to a threshold level and a decision is made as to the presence of the signal of interest based on this comparison.